1. Field of the Invention
The present invention relates to thermoplastic norbornene resin formed articles and substrates for liquid crystal display.
2. Prior Art
Optical parts such as lenses and prisms, information recording medium optical devices such as optical disks, information display medium devices such as liquid crystal displays, and image pickup devices such as CCD have had problems of undesirable light refraction and irregular reflection occurring when the smoothness of their surfaces is poor. Further, to improve the accuracy of input-output of information effected by means of light, it is necessary that the devices have low birefringence or they have low degree of fluctuation of birefringence in the surface. Since birefringence is greatly affected by the unevenness of the surface, good surface smoothness is required also in this regard. Furthermore, optical parts and substrates for optical device preferably have good heat resistance and moisture resistance in consideration of the environment in which they are used.
Previously, the material which has been mainly used in optical parts and substrates for optical device is glass. However, glass is apt to be broken when it is thin, and tends to be too heavy when the thickness is increased. Further, since it is lacking in flexibility, it can hardly be used as a substrate for liquid crystal display having a curved surface. Accordingly, optical resin materials which are highly transparent and free from such disadvantages have come to be used. Such optical resin materials mainly in use are polymethyl methacrylate (PMMA) and polycarbonate (PC). However, PC shows a high birefringence (e.i., a high retardation value), while PMMA shows high water absorption and insufficient heat resistance, so that it is becoming difficult for these resins to meet the demand for increasingly higher quality products.
Recently, thermoplastic saturated norbornene resins, such as hydrogenation products of ring-opening pollers of norbornene group monomers and addition polymers of norbornene group monomers with ethylene, are attracting attention as materials for forming optical parts and substrates for optical device (Japanese Patent Kokai (Laid-open) Nos. 60-26024, 64-24826, 60-168708, 61-115912, 61-120816, and others). Thermoplastic saturated norbornene resins are excellent in transparency, show low birefringence, and also are excellent in heat resistance, moisture resistance, water resistance, electric resistance, solvent resistance, and resistance to chemicals such as acids and alkalis. Further, as compared with glass, they have good strength for their weight and have flexibility. For use in substrates for liquid crystal for example, however, the resins are unsatisfactory in surface smoothness, and the refraction due to surface unevenness will cause the distortion of images. Thus, a high level of surface smoothness of formed articles has come to be required depending on intended uses.
It is true that formed articles having a high level of surface smoothness could be obtained half by chance so long as a surface of very narrow, restricted area was concerned. However, formed articles having a high level of surface smoothness over an extent which can enclose, for example, a circle of 1 cm diameter have not been known in the prior art.